Positive train control (PTC) systems are currently under development in the U.S. and elsewhere. In a PTC system in the U.S., positive knowledge of the location of the end of the train is required since trains must maintain positive length of train awareness. So an accurate, positive measure a train's length (and hence location of the train end) is desirable. Without such a capability, track occupancy circuits will have to be maintained and even expanded from their current density (i.e. more per route mile) in order to shorten headways between trains on a given track segment. Shortening headways between successive trains is one of the operational benefits of migrating from current signal based systems to a PTC system which can allow for more traffic routing and traffic flow flexibility in planning and scheduling.
U.S. Patent Application Publication 2012/0116616 describes a method that continually determines the length of a train operating in a PTC environment. The method uses a line-of-sight vector distance between each end of the train, together with data from an on-train track database, to determine train length.